This invention relates generally to injection semiconductor lasers and more particularly to heterostructure lasers having a unique composition profile providing a high power, low divergence beam.
In the past, five layer heterostructure lasers have been developed for high power, low beam divergence applications. One such device is disclosed by G.H.B. Thompson et al in the Journal of Applied Physics, Volume 47, page 1501 (1976). In these devices, the refractive index profile of the semiconductive layers of the device have been provided with monotonic step type of index profile with the central active layer being the highest refractive index. Specifically, the index profile would be, low-intermediate-high-intermediate-low, vertically through the device. Such a profile allows the optical wave to spread within the device to create a low beam divergence. However, the laser also exhibits an increase in threshold because electrical carriers can be injected into the intermediate index layers rather than being completely confined to the higher index layer as is desirable.
Low beam divergence in the far field pattern may also be provided by a heterostructure injection laser having a very thin active layer. However, in order to confine the optical wave in its propagation in such a device, it is necessary to provide thick cladding layers adjacent to the active layer with high aluminum content. For example, the active layer may be GaAs and the cladding layers may be relatively thick layers of GaAlAs to confine the optical wave. These thick high aluminum content cladding layers lead to high overall thermal resistance thereby reducing the amount of possible input power which, in turn, limits the power outputs necessary for many low beam divergence applications.